Earth's distance from the Sun and its mass help constrain planetary accretion models. Rubie et al. use the initial composition of the mantle as another constraint for accretion simulations, by considering the chemical effects of metallic core formation. The simulations that produce a realistic Earth require that impacting planetessimals span a range of compositions, metalsilicate element equilibration occurs at progressively greater depths as the planet grows, and only a small amount of interaction takes place between the metal impactor core and the proto-mantle. Adding terrestrial mantle composition to accretion simulations provides insights into the most important factors for planetary formation.